For the purpose of improving the corrosion resistance and the coating adhesion of a metallic base material, chemical conversion treatment for forming a chemical conversion film on the surface of a metallic base material by means of a chemical reaction between the material and a chemical conversion treatment solution has been conducted from long ago. The most common chemical conversion treatment to be mentioned first is phosphate conversion treatment based on an acidic aqueous solution of phosphate. A conventional phosphate conversion treatment of a steel material is as follows.
If an acidic conversion treatment solution and a steel material are brought into contact with each other, the steel surface is etched (phenomenon of corrosion). Acid is spent during the etching, so that the pH rises at the solid-liquid interface, and insoluble phosphate is deposited on the steel surface. If zinc, manganese or the like is made coexistent in the conversion treatment solution, zinc phosphate, manganese phosphate, or other crystalline salt is deposited. Deposit films of such phosphates are suitable for a base for coating, and have excellent effects of improving the coating adhesion, suppressing under-film corrosion to greatly enhance the corrosion resistance, and so forth.
Phosphate conversion treatment was put to practical use nearly a hundred years ago, and a variety of improvements have been proposed until today. During phosphate conversion treatment, however, iron dissolves out as a by-product due to the etching of a steel material. The iron is converted in the system into iron phosphate, which is precipitated and periodically discharged from the system. At present, the precipitates (in sludge form) are disposed as industrial wastes, or reused as components of a material for tiles and the like. In recent years, reduction in industrial wastes in themselves is required for a more potent protection of the global environment, and it is earnestly desired to fulfill such requirement by developing a chemical conversion treatment solution or chemical conversion method generating no wastes. In addition, a combined use of a fluoride complex and hydrofluoric acid is necessary for a uniform etching in phosphate conversion treatment, which makes it indispensable to conduct effluent treatment with respect to fluoric components.
Another typical treatment is chromate conversion treatment. Chromate conversion treatment also has a long history of its practical use, and is finding wide application even today in surface treatment of a metallic material, such as an aircraft material, a building material, and a material for automotive parts. The conversion treatment solution to be used for chromate conversion is based on chromic acid comprising hexavalent chromium, and allows a chemical conversion film partially containing the hexavalent chromium to be formed on the metallic material surface. While the chemical conversion film as formed by chromate conversion treatment is excellent in corrosion resistance and coating adhesion, the treatment inevitably requires large-scale effluent treatment equipment because the conversion treatment solution contains hazardous hexavalent chromium, and hazardous fluoric components as well.
[Recently, surface treatment with a chemical conversion treatment solution containing a zirconium compound (hereafter also referred to as “zirconium-based conversion treatment solution”) is attracting attention as the chemical conversion treatment for the metallic material surface that is to be employed instead of phosphate conversion treatment or chromate conversion treatment, and is adapted to reduce environmental impacts. As an example, the following methods are proposed in patent literatures.
Patent Literature 1 proposes a chemical conversion coating agent composed of at least one selected from the group consisting of zirconium, titanium and hafnium, fluorine, and a water-soluble resin.
Patent Literature 2 proposes a chemical conversion coating agent composed of at least one selected from the group consisting of zirconium, titanium and hafnium, fluorine, and at least one selected from the group consisting of an amino group-containing silane coupling agent, a hydrolysate thereof and a polymer thereof.
Patent Literature 3 proposes a chemical conversion coating agent composed of at least one selected from the group consisting of zirconium, titanium and hafnium, fluorine, and an agent for imparting adhesiveness and corrosion resistance.
Each of the zirconium-based conversion treatment solutions as above does not contain chromium, that is to say, has less impact on the environment, and is capable of improving the metallic material surface in corrosion resistance and coating adhesion. The chemical conversion treatment solutions of Patent Literatures 1 through 3, however, contain fluorine, a toxic substance designated, as an essential component. As a recent tendency, ordinances regulating the fluorine content of waste water more severely by defining its permissible values much smaller are put into effect. Since compliance with such ordinances is hardly possible from the viewpoint of not only technology but capital investment, it is a matter of importance and urgency to attain a chemical conversion treatment solution containing no fluorine.
Taking the above problems into account, the technologies as proposed by Patent Literatures 1 through 3 are still far from satisfactory in terms of the reduction in environmental impact.
Patent Literature 4 proposes a chromium-free composition for metal surface treatment, whereupon the chemical conversion film as formed with the proposed composition on the metallic material surface contains a plurality of metallic elements, with at least one metallic element having two or more valences. In the literature, metallic elements Mg, Al, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Sr, Nb, Y, Zr, Mo, In, Sn, Ta and W, as well as oxoates, sulfates, nitrates, carbonates, silicates, acetates and oxalates thereof are described, although neither halides nor halogen-containing compounds are mentioned. Therefore, the proposed surface treatment composition can be considered as fluorine-free. The surface treatment composition, however, is disadvantageous in that it is less stable, does not allow an adequate deposition of metal, and brings about a chemical conversion film with a nonuniform thickness on the metal surface.
Patent Literature 5 proposes the protective film forming method in which a metal protective film obtained from a liquid composition containing (A) at least one selected from among Ti, V, Mn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd and W, (B) at least one selected from among organic acids and/or inorganic acids and/or salts thereof, and (C) fluorine as an optional component is dried without rinsing. The liquid composition contains neither hazardous hexavalent chromium nor a hazardous fluorine compound as an essential component. The protective film forming method as proposed, however, is not appropriate to the surface treatment as base for coating because the metal protective film as dried without rinsing lacks denseness and uniformity and, accordingly, has a poor coating adhesion.
Patent Literature 6 proposes the metal surface treatment method in which a metal surface treatment composition containing zirconium ions and/or titanium ions, an adhesion imparting agent and a stabilizer is used to form a rust preventive film with a high throwing power on a metallic base having a plurality of curved parts before cationic electrodeposition coating. The adhesion imparting agent is (A) a silicon-containing compound, (B) an adhesion imparting ion, or (C) an adhesion imparting resin. The stabilizer is used to prevent components in the rust preventive film from dissolving out during the electrodeposition coating, and is hydroxy acid, amino acid, aminocarboxylic acid, aromatic acid, a phosphonate compound, a sulfonate compound, or a multivalent anion. Fluorine is not an essential component of the surface treatment composition, so that a surface treatment composition containing no fluorine is not focusing attention in itself on its stability. In fact, it was found by the check experiments of Examples 1 and 7 containing no fluorine that iron is stabilized in line with the description, while zirconium cannot be stabilized, leading to precipitates. In other words, it was not possible to form a rust preventive film based on zirconium. The proposed method is thus inappropriate to industrialization.
Patent Literature 7 proposes the metal surface treatment liquid for cationic electrodeposition coating which contains zirconium ions, copper ions and other metal ions, and having a pH of 1.5 to 6.5. The other metal ions are tin ions, indium ions, aluminum ions, niobium ions, tantalum ions, yttrium ions, or cerium ions. The zirconium ion concentration is 10 to 10,000 ppm, the concentration ratio of the copper ions to the zirconium ions is 0.005 to 1 on a weight basis, and the concentration ratio of the other metal ions to the copper ions is 0.1 to 1000 on a weight basis. While fluorine is not an essential component, a fluoride is used in each Example.
Patent Literature 8 proposes the metal surface treatment solution for cationic electrodeposition coating which contains zirconium ions and tin ions, and having a pH of 1.5 to 6.5. The zirconium ion concentration is 10 to 10,000 ppm, and the concentration ratio of the tin ions to the zirconium ions is 0.005 to 1 on a weight basis. While fluorine is not an essential component, a fluoride is used in each Example.
If a zirconium-based conversion agent contains fluorine, a certain amount of fluorine is incorporated into a film of zirconium hydroxide or oxide deposited, which raises the problem of decrease in coating adhesion. Patent Literature 9 proposes a method for setting the fluorine concentration of a chemical conversion film to 10% or less on the atom ratio basis. It is described in the literature that, in order to set the fluorine concentration of the chemical conversion film to 10% or less on the atom ratio basis, a chemical conversion coating agent is caused to contain magnesium, calcium, zinc, a silicon-containing compound, and copper, or the chemical conversion film is heated and dried at a temperature of 30° C. or more, or the chemical conversion film is treated with a basic aqueous solution having a pH of 9 or more to thereby remove soluble fluorine from the film. It, however, is not possible to entirely remove fluoric components adversely affecting the environment and the human body from the chemical conversion film.